One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disk drive. The most basic parts of a disk drive are a disk that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disk drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disk surface. A microprocessor controls most of the operations of the disk drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within a small ceramic block. The small ceramic block is passed over the disk in a transducing relationship with the disk. The transducer can be used to read information representing data from the disk or write information representing data to the disk. When the disk is operating, the disk is usually spinning at relatively high RPM. These days common rotational speeds are 5100 and 7200 RPM. Rotational speeds of 10,000 RPM and higher are contemplated for the future. At such speeds, the very small ceramic block flies on a very thin layer of gas or air. In operation, the distance between the small ceramic block and the disk is very small. Currently "fly" heights are about 0.0003 mm. In some disk drives, the ceramic block does not fly on a cushion of air but rather passes through a layer of lubricant on the disk.
Information representative of data is stored on the surface of the memory disk. Disk drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disk spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disk is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disk drives, the tracks are a multiplicity of concentric circular tracks. In other disk drives, a continuous spiral is one track on one side of a disk drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is composed of many parts that contribute to the performance required to accurately hold the read/write head in the proper position. An actuator includes a pivot assembly, an arm, a voice coil yoke assembly and a head gimbal suspension assembly. A suspension or load beam is part of the head gimbal suspension assembly.
One end of the suspension is attached to the actuator arm. The read/write head is found attached to the other end of the suspension. One end of the actuator arm is coupled to a pivot assembly. The pivot assembly is in turn connected to a servo motor system through the voice coil yoke. The other end of the actuator arm is attached to the head gimbal assembly. The head gimbal assembly allows the read/write head to gimbal for pitch and roll to follow the topography of the imperfect memory disk surface. The head gimbal also restricts motion with respect to the radial and circumferential directions of the memory disk. The suspension is coupled to the actuator arm as part of the mounting support holding the pivot support and coupled to the servo motor. Currently, the pivot assembly is mounted within an opening in a unitized, machined E-block. The E-block includes arms for mounting the suspension on one end and a voice coil yoke on the other end. U.S. Pat. No. 5,283,704 issued to Reidenbach illustrates another actuator system composed of individual components instead of the unitized E-block. This actuator system is "built up" from an actuator arm, spacer rings, a separate voice coil yoke frame assembly, and a separate bearing cartridge. A voice coil is located on the voice coil yoke. The voice coil and magnets attached to the housing of the disk drive form a voice coil motor. The disk drive includes a feedback control loop to enable accurate positioning of the transducer. The disk drive system sends control signals to the voice coil motor to move the actuator arm and the suspension supporting the read/write head across the memory disk in a radial direction to the target track. The control signals indicate to the motor the magnitude and direction of the displacement. The control signals can also be used to maintain the position of the read/write head or transducer over a particular track.
Actuator arms act as spring-mass-damper systems and have resonant frequencies that can degrade the performance of the servo system. Every closed loop servo motor system has a predetermined bandwidth in which resonances occurring within the bandwidth degrade the performance of the servo motor system. The actuator arm is one key source of unwanted resonances. Accordingly, the bandwidths of most servo motor systems are designed such that resonances of the actuator arm occur outside the bandwidth. Each actuator arm has a unique resonance characteristic. Current actuator arms are made of stainless steel, aluminum or magnesium. Suspensions are typically made of stainless steel. The resonance characteristics of the arm are such that the bending modes and torsion modes have frequencies that are within the same frequency range as the suspension and the magnetic storage disk (1 KHz to 8 KHz). Great care must be used when designing an actuator system to prevent alignment of resonance modes that would create very high gains and an unstable servo performance.
In other words, in the presence of a resonance, the transducer or read/write head will vibrate causing it to pass across the desired track. When the resonances of the disk and actuator align or are about the same frequency, the frequency response is amplified so that the amplitude of the vibration is higher and the read/write head travels farther away from the desired track during the track crossings.
The key parameter determining the resonance characteristics of the actuator arm is the stiffness-to-mass ratio of the material. The stiffness-to-mass ratio is about the same for aluminum and stainless steel. Of currently available materials, beryllium and ceramics have significantly higher stiffness-to-mass ratios over that of currently used stainless steel or aluminum. Beryllium is quite expensive and difficult to process while ceramics are prone to crack, particularly under the shock load conditions associated with ball swaging or operational shock loads. As a result, these materials have not become market acceptable for use as actuator arms.
Currently, the predominant method for attaching suspensions to metal arms is a process referred to as ball swaging. As shown in U.S. Pat. No. 4,829,395 issued to Coon et al., the metal arm is provided with an opening. An insert called a swage plate includes a tubular boss. A suspension is typically welded to the swage plate. The boss and attached suspension are positioned on one side of the arm so that they extend into the opening of the arm. A second boss and attached suspension may be positioned on the other side of the arm so that the arm carries two suspensions. An oversized ball is forced through the opening in the one, or both of, the bosses. This forces the material of the bosses into the material of the arm to attach the suspension to the arm.
There are several problems associated with the swaging technique.
One of the problems is that the swage plates are heavy. Due to the weight of the swage plate or swage plates, the tendency in the industry is to go with short arms. The shorter the arm the lesser the rotational moment of inertia. The rotational moment of inertia depends on the weight of the object and the square of the distance away from the axis of rotation. The moment of inertia increases exponentially with the distance from the axis of rotation Increased moments of inertia translate into increased access times for the data. A constant goal of the industry is to lessen the seek times.
Another problem with swaging is that there is industry pressure to reduce the spacing between the disks in a disk stack to produce a shorter disk stack. Swage plates are relatively thick and limit the spacing between the disks of a disk stack having multiple disks. This keeps the height of the disk stack relatively high. In addition, swaged connections are difficult to rework. If an actuator assembly having a swaged connection is found to be faulty, the entire actuator assembly must be reworked (all the suspensions must be removed and replaced). Another shortcoming of swaging is that it is limited with respect to the type of material that the boss can be forced into. Currently, most arms are made of metal and undergo plastic deformation during the swaging process. Arms made of a more brittle or hard material will simply crack or fail upon being shock loaded by forcing the swage ball through the opening in the boss. As a result, swaging limits the type of arm materials to materials capable of plastic deformation, such as metals. The conventional swaging attachment requires a ductile material having a relatively low elastic modulus such as aluminum or magnesium. This enables the boss of the suspension base plate to expand and yield into an interference fit into the mounting hole in the end of the actuator arm. Arms made from ceramic and composite arm materials, which have a relatively high elastic modulus and low ductility, do not lend themselves to swaging process. The ceramic and composite materials exhibit very high compressive strength and lend themselves to attachment methods which clamp the suspension base between the arm and a thin outer conforming sleeve which is held in a state of high internal tension. The appeal for using a ceramic or a composite as an arm material is that some of these materials, although hard and brittle, are lighter and less susceptible to resonant vibration in the frequency of operation of the disk drive. These materials have a lower moment of inertia and also resonate at frequencies beyond the frequencies which normally occur during the operation of the disk drive.
Another problem associated with swaging as well as most other methods for connecting the arm and the suspension is that the electrical connections between the electronics on or near the arm, such as a preamplifier, and the transducer attached to the suspension must be made in a separate step. This takes additional assembly time which also results in increased cost for the suspension as attached to the arm.
Another less common form of attaching a suspension to an arm is by welding. The ceramic and composite arm materials are not compatible with the stainless steel suspension for welding.
The demand for greater track density is increasing steadily, so increasing the performance of the actuator assembly by lowering the mass and increasing the resonance frequencies is becoming a requirement for future systems. There is a need for a disk drive system having an actuator arm which has a higher stiffness-to-mass ratio than actuator arms made of stainless steel, aluminum or magnesium. This would produce a disk drive having lower access times and higher track density. Furthermore, there is a need for a method of attaching a suspension to an actuator that enables different arm materials having a higher stiffness-to-mass ratio than actuator arms made of stainless steel, aluminum or magnesium, to be used. There is also a need for an attachment method that will accommodate materials having different thermal coefficients of expansion.